Dopant-engineered yolk–shell Ni@SiO2 for steam reforming of toluene as a biomass tar model compound: Ce as the key promoter

Efficient tar conversion is essential for reliable biomass gasification and downstream hydrogen production. Yolk–shell catalysts, which confine active metals within an inert shell, show strong resistance to sintering and carbon deposition during tar reforming, but their catalytic efficiency still requires improvement. In this work, promoter-modified yolk–shell Ni@SiO 2 catalysts incorporating Zr, Ce, and Fe were developed for the steam reforming of toluene as a model tar compound. Among the promoters, Ce showed the most significant enhancement, achieving 100% toluene conversion at 650 °C and a steam-to-carbon ratio of 2, with stable performance maintained for 60   h of continuous operation, outperforming the Fe- and Zr-doped catalysts. Mechanistic studies showed that Ce promotion enhanced both activity and stability. TPD-MS results suggested that the introduction of Ce increased the concentration of oxygen vacancies, which promoted H 2 O adsorption on Ce-related sites while preserving metallic Ni sites for toluene activation. Meanwhile, the cracking and reforming temperatures decreased from 315 °C and 398 °C to 220 °C and 262 °C, respectively. In situ DRIFTS identified benzyl alcohol, benzoic acid, and maleic acid as key surface intermediates. These results demonstrate that combining yolk–shell confinement with Ce-induced oxygen vacancy engineering is an effective strategy for stable tar reforming.